Analysing the surface morphology of annealed FTO/ZnS bilayer films: stereometric, fractal, and wettability approaches

The surface micromorphology and roughening of the thermal evaporation-coated FTO/ZnS bilayer thin films annealed at 300, 400, 500, and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$550\,^\circ{\text{C}}$$\end{document}550∘C for 1 h have been studied. AFM images of the prepared samples were analysed by the MountainsMap software, and the effects of the annealing temperature on the surface texture of the FTO/ZnS thin film’s surface were investigated. Stereometric and advanced fractal analyses showed that the sample annealed at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$500\,^\circ{\text{C}}$$\end{document}500∘C exhibited greater surface roughness and greater skewness and kurtosis. This film also has the most isotropic surface and exhibits the highest degree of heterogeneity. Also, despite the decrease in surface roughness with increasing temperature from 500 to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$550 \,^\circ{\text{C}}$$\end{document}550∘C, the fractal dimension tends to increase. The static water contact angle measurements indicate that the film annealed at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$500 \,^\circ{\text{C}}$$\end{document}500∘C exhibits higher hydrophobicity, which can be attributed to its greater topographic roughness. Our research indicates that the surface morphology of FTO/ZnS bilayer thin films is influenced by the annealing temperature. Changing factors such as roughness, fractality, and wettability parameters to help improve surface performance make the FTO/ZnS bilayer suitable for application in electronic and solar systems.


Sample preparation
The FTO/ZnS bilayer system was fabricated on a glass substrate.For this purpose, the glass substrate was washed with soap and water.Then, using an ultrasonic apparatus, each sample was washed in ethanol, methanol, dichloromethane, isopropanol, acetone, and deionized water for 5 min and dried under a nitrogen flow.Preparation of the F-doped SnO 2 (FTO) solution was conducted as follows: First, 5.93 mmol of SnCl 2 .2H 2 O was dissolved in 20 ml of ethanol in a closed container.The solution was continuously stirred until all the SnCl 2 .2H 2 O was completely dissolved in ethanol.Then, 5.26 mmol of NH4F was dissolved in 20 ml ethanol, in another container.After stirring thoroughly, both solutions were mixed at room temperature.The combined solution was allowed to stir for 3 h to ensure complete mixing, and the resulting solution became homogeneous and stable.Next, the final solution was sprayed onto glass substrates when set on a rotating hot plate.The substrate temperature and solution flow rate were maintained at 300 • C and 1 ml/min, respectively.The distance between the nozzle and the substrate was approximately 25 cm.The thickness of the fluorine-doped SnO 2 film estimated by a Dektak profilometer was about 192 nm.For the deposition of the second layer, ZnS material was purchased from Sigma Aldrich as a source material with a purity of 99.99% .A tantalum crucible was applied to evaporate the target material in the thermal evaporation device.The base pressure of the vacuum chamber was 9 × 10 −6 mbar, and the pressure during the deposition process was 4.1 × 10 −5 mbar.We controlled the thickness and deposition rate and measured values of 200 nm and 0.1 nm/s, respectively, with a quartz crystal system.Also, the substrate temperature was kept at room temperature during the growth process.Annealing was performed to stabilize the layers after deposition.The bilayer system was annealed in a thermal annealing furnace at 300, 400, 500, and 550 • C for 90 min and labeled S300, S400, S500, and S550, respectively.

Characterization
To determine the exact doping level, Micro X-ray fluorescence (XRF) analysis was performed to study the elemental composition of the F-doped SnO 2 powder, using the M4 Tornado model (Bruker, Germany).The XRF results showed that various elements including Sn (70.86 wt%), O (18.97 wt%), F (9.86 wt%), and traces of other impurities such as Ca (0.083 wt%), Zn (0.061 wt%), k (0.079 wt%), Fe (0.039 wt%) and Al (0.048 wt%) were identified in the sample.Topographic images were obtained using atomic force microscopy (AFM) in tapping mode/non-contact mode with the Park NX-10 model (Park Systems, South Korea).The AFM measurements were carried out at room temperature using cantilevers with a 10 nm silicon tip radius.The 3 µm × 3 µm and 256 × 256pixel AFM images were obtained with scan rates of 1.0 Hz .The relative humidity (RH) and tempera- ture were monitored using a digital hygrometer/thermometer inside the AFM chamber.The amount of relative humidity was constant during the characterization process.Measurements were performed at room temperature ( 22 ± 1 • C ) and relative humidity ( 45 ± 1% ).The surface wettability of the films was measured with a contact angle (CA) apparatus using a water droplet volume of about 4µL with the SEO model (South Korea).

Data processing and surface analysis
The commercial software MountainsMap ®premium ver.10.1.10571(Digital Surf, Besançon, France) 36 according to the standard ISO 25178-2:2012 was used for data processing and presentation of much of the results in this study.MountainsMap is a powerful metrological tool for surface texture analysis.Additionally, algorithms for computing advanced fractal parameters were developed in MATLAB via the box-counting method.

Advanced stereometric analysis
Stereometric analysis has been extensively applied across distinct fields, such as biology 37 , medicine 38 , materials science, and geology.It quantitatively measures the volume, surface area, length, and other structural parameters of intricate three-dimensional objects.Stereometric parameters were employed to assess the correlation between the topographic patterns and microtextures of the film surface 39 .The 3D-AFM images in Fig. 1  The maximum values of the root mean square of height (S q ) and mean roughness (S a ) were observed in sample S500, with values of 15.21 ± 0.78 nm and 11.51 ± 0.40 nm, respectively.Furthermore, by increasing the anneal- ing temperature from 300 to 500 • C , S q increases, and then S q decreases at an annealing temperature of 550 • C .This happens because the annealing process involves the interaction of various factors, such as grain growth, island coalescence, and crystallization 40 .A similar roughness trend, maximum peak height and valley depth, S p , and S v , and sum of the maximum height, S z , were observed for sample S500.This result suggests an increase in the spatial complexity of sample S500, specifically emphasizing higher topographic irregularity.The positive  www.nature.com/scientificreports/value of surface skewness (S sk ) affirms the dominance of the peak.The surface kurtosis (S ku ) displays different shapes of height distributions for samples S300 and S400 compared with samples S500 and S550.A kurtosis above three (S ku > 3 ) indicates a spiky or sharply peaked surface which is visible for all samples 41 .The maximum and minimum values of S ku in samples S500 ( 8.259 ± 0.405 ) and S300 ( 3.119 ± 0.129 ), respectively.Figure 1(a 2 -d 2 ) presents the height distribution and Abbott-Firestone curve associated with relevant analysed samples.Functional parameters pertain to the distribution of heights and the cumulative curve, commonly recognized as the Abbott-Firestone curve.These parameters serve the purpose of characterizing the functional behavior of a surface, particularly in contexts such as wear, lubrication, and contact.They operate globally, encompassing field parameters, and can be viewed as statistical metrics.Owing to their association with height distribution, bearing ratio parameters can be readily extended to surfaces.The depth histogram illustrates the density distribution of surface data points.The horizontal axis represents the bearing ratio (in %), while the vertical axis indicates the depth, accompanied by the Abbott-Firestone curve.The histogram associated with a sample annealed at 500 • C has a normal distribution, whereas samples annealed at 300 and 400 • C have a left-skewed distribution.
The Abbott-Firestone curve represents the bearing ratio curve, signifying, for a specific depth, the percentage of material traversed relative to the covered area.This function serves as the cumulative function of the amplitude distribution function.The Abbott-Firestone curves of samples S300 and S400 exhibit an approximately S-like shape, implying that the surfaces are influenced by a height distribution that approximates Gaussian characteristics, making the surfaces more uniform 42 .The arithmetic mean peak curvature (S pc ) is greater for S500, revealing that this surface has rough peaks with more rounded shapes.
The polar spectrum graphs in Fig. 2 represent the texture directions, whose corresponding results for each sample are summarized in the table on the right-hand side of the figure.The highest and lowest values of the isotropy parameter are related to sample S500 ( 94.06% ) and sample S400 ( 81.20% ), respectively.The first, second, and third directions are similar for samples S300, S400, and S550 ( 90.00 • , 135.00 • , and 153.50 • , respectively).Sample S500 exhibited the highest value in the first direction ( 135.00 • ) and the lowest values in the second and third directions ( 44.99 • , 90.00 • ).
This study was performed to analyse the surface complexity of the samples.One approach to emphasize the image texture involves representing the 3D surface of thin films.More sophisticated qualitative rendering methods are available to highlight surface texture and simulate flooded images using contour lines and microfurrows.The flow area and fluid entrance are depicted by the microfurrows generated on the bilayer FTO/ZnS surfaces (see Fig. 3(a 1 -d 1 )).The furrow images of all the samples are uniform.Lighter areas display rougher furrows, while darker ones display valleys.The quantitative features of the furrow map, including the maximum furrow depth, average furrow depth, and average furrow density, were extracted and are presented in Table 1.The higher maximum furrow depth was observed in sample S500 ( 64.19 ± 2.95 nm), suggesting the greatest roughness of the S500 surface.Sample S500 also shows the same behavior for the average furrow depth (AFD) and average furrow density (AFD sty ) with values of 26.97 ± 1.30 nm and 86163 ± 2783 cm/cm 2 , respectively.The approxi- mate similarity of the depth of the channels for samples S500 and S550 shows that the samples have almost the same wettability.This also applies to samples S300 and S400.The high-quality photo simulation of the flooded image through the contour lines is presented in Fig. 3(a 2 -d 2 ).This behavior illustrates the association between the microtexture and annealing temperature.Lighter regions correspond to rougher furrows, whereas darker regions are associated with valleys 43 .Figure 4(a 1 -d 1 ) shows Abbott-Firestone curves of thickness parameters that are connected to the core of the surface's microtexture.The other quantitative values of the S k parameters are listed in Table 1.The highest thickness of the surface central part in the z-direction, core height (S k ), belonged to sample S500 ( 35.18 ± 1.86 nm), while sample S300 presented the lowest value of 19.16 ± 0.85 nm.The S500 and S400 samples exhibit the highest ( 10.49 ± 0.38 nm) and lowest ( 5.83 ± 0.24 nm) values for reduced peak depth (S vk ).In addition, the average value of the peak/valley material portion (Smr1/ Smr2) is the percentage of the material ratio curve that corresponds to the upper/lower limit of the roughness core profile S k , which does not fluctuate.The height parameters have broad applicability and are frequently employed in practical applications, specified in units of nm or µm .Conversely, volume parameters are infrequently utilized in the context of surface wettability studies.It seems that the void volume can be a suitable parameter for characterizing the extent of bubbles on surfaces.This choice is grounded in the understanding that the void volume parameter indicates the amount of air that would occupy the surface (normalized to the measurement area) within the specified material ratio values.These void volume measurements are derived from the surface roughness parameter, S a , described in detail as a function-related parameter in 28 .Figure 4(a 2 -d 2 ) shows the Abbott-Firestone curve, which encompasses all volume-related parameters obtained through ISO 25178:2012 (Table 1).All functional parameters (volume) had the highest values for sample S500.

Fractal characterization
The power spectral density (PSD) has been a valuable mathematical tool for analysing surface patterns with varying dominant wavelengths.A survey of the PSD involves establishing a correlation between the vertical amplitude and the spatial frequency of surface features.This analytical approach offers valuable insights into the characteristics of the surface microstructure 44 .Figure 5 displays the linearized graphs of the average PSD spectrum.All samples exhibit the same wavelength of 0.5309 µm , maintaining a constant zoom factor of × 4. Additionally, sample S500 demonstrated the highest amplitude (0.2082 nm), while sample S300 (0.1457 nm) and sample S550 (0.1822 nm) displayed intermediate values, with the lowest amplitude recorded for sample S400 (0.000 nm).The highest value of the dominant wavelength appears in sample S550 ( 0.0467 µm ), followed by sample S300 ( 0.0425 µm ).Samples S400 and S500 have the same thickness ( 0.0170 µm ).The maximum ampli- tudes are 3.174, 4.737, 20.220, and 15.340 nm for samples S300, S400, S500, and S550, respectively.The fractal dimension, D f , was calculated using the enclosing boxes method (EBM) for extra-fine resolutions, as presented in Fig. 5 (top left).The EBM involves partitioning the area into smaller sections with a width ε and computing the field Aε for all the fields covering the entire area.This process is iterative, where the field width is adjusted to plot ln(Aε)/ln(ε) .A line is fitted to the plot to estimate the fractal dimension by the least squares method.The magnitude of the slope of this fitted line serves as the estimation for the D f .The graph illustrating the calculated volume (for surfaces) is plotted as a function of the scale 45 .The fractal dimension values of the samples were 2.64 ± 0.08 (S300), 2.71 ± 0.08 (S400), 2.74 ± 0.08 (S500), and 2.77 ± 0.08 (S550).Generally, a higher D f in rough fractal surfaces indicates a more complex geometry and irregular shape of the S550 surfaces.The fractal dimension of the sample surfaces was determined using three distinct resolutions, specified by the number of

Advanced fractal analysis
In pursuit of a more quantitative depiction of surface microtexture behavior, we employed the fractal dimension with two novel fractal parameters, namely, fractal succolarity, and lacunarity.In some cases, two images may display the same fractal dimension yet exhibit distinct lacunarity or have identical lacunarity but divergent succolarity.Additionally, a variety of other combinations of results may arise.Fractal socularity (FS) represents the Greater succolarity values are associated with diminished percolation, yet when these values approach 0.5, there is an increased likelihood of fluid penetration 47 .When the succolarity is below 0.5, the surface exhibits low percolation.The observed low succolarity values suggest that the roughness might not impact the wettability 48 .The trend of the changes in the contact angle of the samples is consistent with the variations in surface roughness.Therefore, S550 likely presented the highest succolarity value due to its lower porosity.However, the values were close to 0.5, indicating an ideal surface percolation value, as suggested by Melo and Conci 49 and de Oliveira et al. 47 .Incorporating fractal succolarity and lacunarity can prove valuable for the advanced characterization of surface images 50 .Lacunarity measures the distribution of gaps or voids in a pattern, and the box-counting method is a common approach for characterizing fractal patterns.The calculation of FL follows the model described by Lucena et al. 51 : We derived the lacunarity coefficient ( β ) from the lacunarity curve using the following equation to acquire information regarding the surface texture homogeneity 52 : where FL(r) is the lacunarity distribution, β is the lacunarity coefficient, and r is the box size.Lacunarity enables discrimination between sets with identical fractal dimensions but distinct textures 53 .The results summarized in Table 2 show that the lowest value of β is observed for sample S300, which suggests that the distribution of gap sizes on the surface is the most uniform 54 .Additionally, the S500 sample exhibited the highest grade of heterogeneity.Lacunarity is indicative of heterogeneity, whereas succolarity signifies anisotropy 55 .

Wettability behavior
Advanced fractal analysis facilitates understanding the mechanisms of nanostructure formation and provides insights into the wettability of thin film surfaces.This mathematical tool is a decisive parameter governing contact angle dynamics 44 .An increase in wettability can be a consequence of the fractal nature of the surface.The wetting degree of the surface is determined by measuring the contact angle (CA) when a solid and liquid interact.Any solid surface may be wettable(repellent) for a liquid if the contact angle is > 90 • (< 90 • ).The effect of annealing temperature on the wettability behavior of the FTO/ZnS bilayer system is studied based on the concept of fractal properties.Regardless of gravity, a drop of liquid on a flat surface assumes the shape of a spherical cap.The interfacial free energy per unit area at the solid-liquid ( γ sl ), liquid-vapor ( γ lv ), and solid-vapor ( γ sv ) interfaces   www.nature.com/scientificreports/determine the wettability of the surface.The relationship between the contact angle of a liquid droplet with a plane surface and the interfacial free energy is expressed by Young's basic equation 56 : Here θ 0 = 0 corresponds to complete wetting when the liquid and solid surface energies ( γ lv and γ sv ) are low and high, respectively.The surface will be hydrophilic (hydrophobic) when the contact angle is > (<) 90 • , and γ sv > (<) γ sl 57 .Young's assumption neglected surface roughness, rendering his model equation incapable of accurately describing wetting behaviors on rough surfaces. 58.For the first time, Wenzel (in 1936) proposed a theoretical model to describe the contact angle of a liquid drop on a rough solid surface (Young's modified equation).Wenzel showed that the effective area of a rough solid surface increases by a roughness factor of r (the ratio of the actual area of a rough surface to the projected area) 59 : where the contact angle on a smooth surface is denoted by θ Y (the Young contact angle), and θ W is the apparent contact angle on the rough surface.r is called the roughness factor or roughness ratio.As the roughness of the surface increases, the surface becomes more wettable.r = 1 refers to a smooth surface, while r > 1 corresponds to a rough surface 44 .Originally, surface roughness amplifies the wetting or nonwetting characteristics of a substrate (increasing surface roughness consistently improves either hydrophilicity or hydrophobicity 60 ).When water enters a hydrophobic specimen, the increase in roughness leads to an increase in the contact angle increasing the hydrophobicity of the surface 61 .The relationship between the surface fractal dimension D f and the r factor is expressed by a first-order approximation as 62 : Here, D f (2 < D f < 3) is the fractal dimension, where L and l denote the upper and lower limit cut-offs/lengths of fractal behavior, respectively, and the associated contact angle corresponding to the classic Wenzel model is expressed as follows 63 : Hence, the surface wettability can be controlled by modifying the fractal dimension, as proposed by Eq. (7). Figure 7 shows the variation in the contact angle, fractal dimension, and roughness with annealing temperature.The observed contact angles for samples S300, S400, S500, and S550 were 61.0 ± 2.4 • , 77.5 ± 3.1 • , 103.4 ± 4.1 • , and 84.6 ± 3.4 • , respectively.Higher annealing temperatures increase the surface complexity, resulting in increased hydrophobicity.The contact angle measurements suggested that the surfaces of the S500 and S550 samples are hydrophobic, while the surface of the S300 is is highly hydrophilic, as shown in Fig. 7.A surface with a higher fractal dimension will repel water more strongly.As a result, surfaces with higher fractal dimensions yield larger contact angle values.Surfaces with a small fractal dimension do not exhibit significant water-repelling properties, resulting in small contact angle values on such surfaces 64 .

Conclusions
In conclusion, our study focused on the precise preparation process and characterization of FTO/ZnS bilayer thin films on glass substrates by investigating their 3D-surface morphology, statistical properties, fractal, and wettability behavior.The power spectral density (PSD) is a valuable tool for analysing surface patterns, offering correlations between vertical amplitude and spatial frequency.The calculation of the fractal dimension, employing the enclosing boxes method, exhibited intricate geometries.Topographic characterization through atomic force microscopy provided a detailed understanding of surface morphology from advanced fractal and wettability aspects.We explored the interplay between surface roughness, fractal properties, and wettability, revealing that higher annealing temperatures contribute to increased surface complexity and hydrophobicity.The observed variations indicated a significant impact of annealing temperature on fractal behavior, with higher annealing temperatures leading to increased fractal dimension and fractal succolarity.Following that, as the annealing temperature increases up to 500 • C , the increase in roughness points to increasing lacunarity (heterogeneity); further increasing the temperature to 550 • C decreases the roughness and lacunarity.Wettability analysis, through the modified Wenzel equation, employing static water contact angle measurements, revealed a similar trend for hydrophobicity and roughness with annealing temperature, with the highest values of roughness ( 15.21 ± 0.78 nm) and contact angle ( 103.4 ± 4.1 • ) obtained at 500 • C .F-doped SnO 2 /ZnS thin films exhibited unique fractal and wettability characteristics, making them highly valuable in applications where surface properties are crucial.Our comprehensive approach, integrating stereometric characterization techniques with advanced fractal analysis, provides a detailed understanding of these bilayer thin films.These findings are valuable for fundamental research in surface science and have practical implications for enhancing hydrophobic coatings, sensor technology, and advanced electronic devices.

Figure 6 .
Figure 6.Results of the surface fractal dimension ( D f ) for three considered options determined by the EBM method for FTO/ZnS thin films annealed at various temperatures.

Figure 7 .
Figure 7. Contact angle, fractal dimension, and roughness of FTO/ZnS thin films versus annealing temperature.
that different rough surfaces are formed due to different annealing temperatures.Moreover, Table1summarizes the statistical parameters deduced from 3 µm × 3 µm scanning square areas of bilayer FTO/ZnS AFM images.

Table 1 .
The height and statistical parameters of the FTO/ZnS thin films, according to ISO 25178.
*Samples without significant difference according to one-sample t-test: p-value > 0.05